The present invention relates to the production of finely divided, stoichiometric titanium disulfide and more particularly to a process for directly reacting metallic titanium with elemental sulfur to form finely divided, stoichiometric titanium disulfide.
Stoichiometric titanium disulfide has a lamelliform structure in which each lamella is a hexagonal layer of titanium metal surrounded on either side by a layer of sulfur atoms, and the lamella are bound to each other by weak van der Waals forces. The individual lamella being held together merely by van der Waals forces can be readily intercalated by ions and molecules. The ease with which titanium disulfide can be intercalated makes it a material that has great potential as a cathode material in secondary batteries because the rapid rate of intercalation will permit rapid discharge at battery cathodes.
The rate at which titanium disulfide can be intercalated is adversely affected by non-stoichiometry, particularly if titanium disulfide is titanium-rich, because the excess titanium is found between the individual lamella where it interferes with the mobility of the intercalating species. The particle size of the titanium disulfide also has an effect on the rate of intercalation. The rate of intercalation is greater in stoichiometric titanium disulfide having a finer particle size because the finer crystallites expose far greater areas of inter-lamella spacings into which the intercalating species can enter the crystallites and the lengths of intra-crystalline diffusion paths are significantly reduced. Thus, if titanium disulfide is to be used as a cathode material in secondary batteries, it is advantageously stoichiometric and finely divided.
Titanium disulfide has been prepared by at least three methods. Titanium halides have been reacted with hydrogen sulfide to form titanium disulfide, but this method has the disadvantage that the titanium disulfide is contaminated with the halide species which can interfere with intercalation. Somewhat related to the previous method is the halide transport method in which titanium metal, elemental sulfur and small amounts of a halide, preferably iodide or bromine, are added to a reaction vessel which is evacuated, sealed and then differentially heated to produce titanium disulfide. The halide transport method has the disadvantage of contaminating the titanium disulfide with the halide transport species and of producing titanium disulfide having larger than desired crystallites. Titanium disulfide has also been prepared by directly reacting titanium metal with elemental sulfur by heating the reactants to a reaction temperature, for example about 800.degree.C., and holding for long periods of time to insure substantially complete reaction between the titanium and the elemental sulfur.